Coating agent for reducing the soiling process of facades

ABSTRACT

The present invention relates to a transparent coating agent that comprises a layered silicate capable of forming a colloidal gel in the presence of water. This invention also relates to the preparation of this agent and to its use for reducing the soiling process of facades.

This is a 371 filing of PCT/EP98/03879 filed Jun. 25, 1998.

The present invention relates to a transparent coating composition basedon a sheet silicate, to its preparation and to its use for reducing thesoiling tendency of facades.

Facades, like any other surface, become soiled over time owing todeposits from the air onto the surface. This soiling is manifested ingraying. Using light microscopy and SEM investigations on facadecoatings which have been weathered under open-air conditions it has beenfound that the soiling consists essentially of inorganic particles witha size of up to 10 μm along with small fractions of soot.

U.S. Pat. No. 2,877,142 discloses the dirt repelling action of inorganicsols, such as silica sols, on surfaces. The use of silica sols onfacades is hindered by their low viscosity. In the DIN 4 mm flow cup,these sols have a viscosity of 14.3 s (23° C.). Application techniquesused are brushing, rolling and spraying. When applied at a rateappropriate to that in practice of about 50-250 g/m², preferably 50-150g/m², the silica sols run on vertical faces such as facades. It istherefore impossible to obtain the even coat thickness that is requiredfor the coating to have a uniform dirt repelling effect. At applicationrates of from 50 g/m² to 250 g/m², splashes and water runs occur whichprevent an even coating. A thixotropic or at least pseudoplasticconsistency, which permits brushing tools to be used without problems,is not possible with sols. Such a consistency is obtained if in the DIN4 mm flow cup efflux times of more than 100 s at least are measured.

The object of the present invention, then, is to provide a transparent,readily processable, color-neutral and at least pseudoplastic coatingcomposition which permits an even application to facades and at the sametime reduces their soiling tendency and the attenuation of gloss.

It has been found that only a few selected materials are compatible withthis object for the coating composition of the invention. The choice ofa suitable rheological material has a critical influence on the effectof the coating material. For instance, organic gel formers such ascellulose ethers, polyacrylates and polyurethane thickeners remove theactivity of the silica sols in reducing soiling. In the case ofinorganic gel formers, the soiling-reducing effect of the silica sols isnot removed. For instance, with coating compositions based on silica soland synthetic or natural sheet silicates a marked reduction was found inthe soiling tendency. However, natural inorganic sheet silicates areunsuited to the set object owing to the lack of color neutrality of thecoating composition. The observed yellowishness of the coatingcomposition did not permit color-neutral drying. The yellowishnessoriginates from the iron inclusions in the sheet silicates. Sheetsilicates from all known deposit regions have iron inclusions. Inaddition, the natural sheet silicates are not completely transparent.The average particle sizes of the known natural sheet silicates arewithin the micrometer range. Pyrogenic silicas are likewise inorganiccompounds, but cannot be processed with silica sol and water to give ahomogeneous coating composition and so do not meet the set object.Precipitated silicas, again, are inorganic compounds, but have averageparticle sizes of more than one micrometer. Consequently, the coatingcomposition prepared from them using silica sol and water does not dryto a transparent film.

It has surprisingly been found that synthetic inorganic sheet silicateswhich give a transparent gelatinous paste with water possess both per seand in combination with silica sols, a dirt repelling action and alsocomply with the other conditions of the set object. They are thereforesuitable as coating compositions for the stated set object. Theviscosity in this case is preferably established so that an evenapplication of from 50 g/m² to 250 g/m² is possible. The consistency israised to such an extent that in the DIN 4 mm flow cup efflux times ofmore than 100 s at least are measured. It proves even more favorable ifthe medium can no longer be found to have any fluidity. The resultingcoating material shows thixotropic properties. Application to substratescan now be made, even using brushing tools, without splashes or runs.The resulting coating is transparent and does not show any color changesof the substrate.

The invention accordingly provides a coating composition comprising atleast one sheet silicate which forms a colloidal gel in water. Thecoating composition is transparent and the sheet silicate used ispreferably a nanoscale silicate having an average particle size of from5 to 800 nm, preferably from 25 to 500 nm, in particular from 100 to 400nm.

Sheet silicates suitable in accordance with the invention are availablecommercially; suitable examples include SKS—20/Saponite (Hoechst AG,Frankfurt, Germany), SKS—21/Hectorite (Hoechst AG; Frankfurt, Germany),Optigel® SH (Sud Chemie AG, Munich, Germany) and Laponite® RD (DeutscheSolvay Werke GmbH, Solingen, Germany). Mixtures of these sheet silicateswith one another are also suitable. All of these colloidal gels, whichhave been prepared from water with synthetic inorganic sheet silicates,preferably having a particle size of below 1 micrometer, meet the objectof the invention. The transparent coating composition also features thedesired properties when other, at least colloidally soluble inorganicsubstances (for example, silica sols) are present.

In the text below, the invention is illustrated with reference toexamples.

Various coating compositions were prepared and their action was testedon the substrate and in the soiling test. The coating compositionscontained different gel-forming substances along with silica sols andwater. The preparation and testing are described in detail.

EXAMPLE 1

Composition: 6.7 g of Klebosol 30 N 12 (Société Francaise Hoechst/silicasol)

91.3 g of water

2.0 g of Laponite RD

38 g of water were introduced initially. The Laponite was added withstirring on a dissolver at 2700 rpm. After 10 minutes of stirring, thebase gel was prepared. This mass was aged at 23° C. and 50% relativeatmospheric humidity for 16 h. Then 53.3 g of water and the silica solwere added. Finally, the mass was stirred on a dissolver for 5 minutes.

The colloidal gel, with about 50 percent by weight of synthetic resindispersion as binder component, was then coated onto a polymerdispersion facade paint. The application was made with brushing tools,with no splashes or runs. The result was a transparent, imperceptible,even coat. The application rate was 60 g/m².

In parallel, the coating composition was applied to a synthetic resindispersion render which had been dried for 24 hours. The composition wasapplied using the same tools. An even coat without water runs wasobtained. The application rate was 150 g/m².

EXAMPLE 2

Composition: 95.0 g of water

5.0 g of Laponite RD

The water was introduced initially. The Laponite was added with stirringon a dissolver at 2700 rpm. After 10 minutes of stirring, the base gelwas prepared. The coating composition was finally aged at 23° C. and 50%relative atmospheric humidity for 16 h.

The colloidal gel, with about 50 percent by weight of synthetic resindispersion as binder component, was then coated onto a polymerdispersion facade paint. The application was made with brushing tools,with no splashes or runs. The result was a transparent, imperceptible,even coat. The application rate was 70 g/m².

In parallel, the soiling tendency of the coating composition was tested.For this purpose, a synthetic resin emulsion paint with about 50 percentby weight of synthetic resin dispersion as binder component wasknife-coated onto the surfaces of Leneta sheets. The application ratewas 250 g/m². The sheets were stored under standard climatic conditions(23° C./50% relative atmospheric humidity) for 48 hours and finally at40° C. for 24 hours. The coating composition was subsequently applied tothese samples using a paintbrush. The application rate was 100 g/m². Thecoatings were then dried at room temperature. Subsequently, the samplewas covered with dust. The model substance used was fly ash. The dustnot adhering to the sample was removed using compressed air. As areference for the test, the surface of a synthetic resin emulsion paintwas used. Both samples were tested visually. The coating compositionexhibits a distinct reduction in the amount of dirt adhering to thesynthetic resin dispersion surface.

EXAMPLE 3

Composition: 6.7 g of Klebosol 30 N 12 (Société Francaise Hoechst/silicasol)

91.3 g of water

2.0 g of gel former (see example)

38 g of water were initially introduced. The gel former was added withstirring on a dissolver at 2700 rpm. After 10 minutes of stirring thebase gel was prepared. This mass was aged at 23° C. and 50% relativeatmospheric humidity for 16 h.

Then 53.3 g of water and the silica sol were added. Finally, the masswas stirred by a dissolver for 5 minutes.

Gel formers used were the following substances:

From the class of precipitated silicas:

Kieselsäure 320 DS (Degussa, Hanau, Germany);

Syloid® ED 3 (Grace, N.Y., USA)

From the class of natural sheet silicates:

Bentone® CT (Rheox); Bentone LT (Rheox);

From the class of pyrogenic silicas:

Aerosil® LR 972 (Degussa);

From the class of synthetic sheet silicates:

Laponite RD (Deutsche Solvay GmbH); Optigel SH (Süd Chemie);

SKS-20/Saponite (Hoechst); SKS-21/hectorite (Hoechst);

From the class of polyacrylates:

Latecoll® D (BASF, Ludwigshafen, Germany);

Rohagit® SD 15 (Roehm); Carbopol® (Goodrich);

From the class of cellulose ethers:

Walocel® XM 30,000 PV (Wolff-Walsrode, Walsrode, Germany);

Tylose® MG 30,000 YG 8 (Hoechst);

From the class of polyurethane thickeners:

Coatex® BR 125 (Coatex); Acrylsol® RM 8 (Rohm & Haas); Coatex AC 668(Coatex); Coapur® 5035 (Coatex); Rheolate® 278 (Rhoex).

During the preparation of the coating composition, the incorporation wasassessed. The Aerosil LR 972 could not be stirred in. Bentone CT andBentone LT gave a yellowish tinge to the coating composition, so that itwas not possible for color-neutral and transparent coatings to beformed.

The assessment of the transparency and of the color neutrality was madeon a green synthetic resin emulsion paint with about 50 percent byweight of synthetic resin dispersion as binder component. The paint wascoated onto a fiber cement slab, stored in the laboratory for 3 days andthen coated with the coating composition. After the drying of thecoating at room temperature, the appearance of the surface was comparedwith a standard. In this case it was found that Kieselsäure 320 DS andSyloid ED 3 lacked transparency and color neutrality. With the gelformers Coatex BR 125, Rheolate 278, Coatex BR 100, Acrylsol RM 8,Coatex AC 668 and Coapur 5035, the slabs were found to have a mottledappearance in comparison with the standard following watering andsubsequent drying. The polyacrylates Latecoll D; Rohagit SD 15 andCarbopol also showed this phenomenon.

In order to test the soiling tendency of the coating compositions, asynthetic resin emulsion paint with about 50 percent by weight ofsynthetic resin dispersion as binder component was knife-coated onto thesurface of the Leneta sheets. The application rate was 250 g/m². Thesheets were stored under standard climatic conditions (23° C./50%relative atmospheric humidity) for 48 hours and finally at 40° C. for 24hours. The coating composition was then applied to these samples using apaintbrush. The application rate was 100 g/m². The coatings weresubsequently dried at room temperature. Then, one sample of each coatingcomposition was watered for 15 hours. This sample and a further,unwatered sample of the coating composition were wetted and then soiled.The model soiling substance used was fly ash. The graying wassubsequently quantified. This was done by determining the brightnessvalues in accordance with DIN 53778 using a color difference measuringapparatus. The difference between the brightness values of the soiledsamples and unsoiled reference samples gave the measure of the soilingtendency. In comparison to the uncoated soiled synthetic resin emulsionpaint, a reduction in the soil uptake was found in the case of LaponiteRD; SKS-20/Saponite, and SKS-21/hectorite. No reduction in the soilingtendency was found with the use of gel formers such as Tylose MW 30,000YG 8 and Rohagit SD 15. An increase in the soiling tendency relative tothe uncoated synthetic resin emulsion paint was found with Latecoll D,Walocel XM 30,000 PV and Coatex BR 100.

The attached table summarizes the results by gel former type.

Properties of the transparent coating compositions as a function of thegel former Trans- Proc- Color Soil Gloss Product Type parency essingneutrality adhesion attenuation Precipitated no good no — yes silicasNatural sheet no good yellowish no no silicates Polyacrylates mottledgood no yes no Cellulose — good — yes — ethers Pyrogenic — poor — — —silica Polyurethane mottled good mottled yes — thickener Synthetic sheetyes good yes no no silicate

What is claimed is:
 1. A transparent coating composition comprising atleast one synthetic sheet silicate and at least one silica sol whichform a colloidal gel with water.
 2. The coating composition as claimedin claim 1, wherein the sheet silicate is a nanoscale sheet silicate. 3.The coating composition as claimed in claim 1, wherein the sheetsilicate has an average particle size of 5 to 800 nm.
 4. The coatingcomposition as claimed in claim 1, which in the DIN 4 mm flow cup has anefflux time of at least 100 s.
 5. The coating composition as claimed inclaim 1, which possesses no fluidity.
 6. The coating composition asclaimed in claim 1, which is thixotropic.
 7. The coating composition asclaimed in claim 1, which comprises no organic gel formers.
 8. A processfor preparing a transparent coating composition comprising the steps of:adding water to a synthetic sheet silicate and a silica sol which form acolloidal gel with water and processing the mixture to a gel.
 9. Acoating formulation for the purpose of dirt repellence comprising atransparent coating composition including at least one synthetic sheetsilicate and at least one silica sol which form a colloidal gel withwater.
 10. A dirt repelling coating comprising at least one syntheticsheet silicate and at least one silica sol which form a colloidal gelwith water.
 11. A coating formulation comprising a pigment, a syntheticsheet silicate, and a silica sol which form a colloidal gel with water.